The present invention generally relates to spot welding guns and more particularly to a servo-pneumatic modular weld gun system having freely programmable closed loop position and pressure control. The present invention is well suited to rapid assembly line welding applications, particularly in the automotive industry.
Resistance or spot welding joins middle sheets or structures through heat produced by resistance to the flow of electrical current. The metal structures to be welded are typically clamped together with rod shaped electrodes applying pressure on opposite sides. An electric current passing between the electrodes meets resistance when it flows across the metal structures producing heat that melts the metal and welds them together.
Conventional spot welding guns utilize one or more pneumatic cylinders to extend and retract a moveable electrode and to apply the pinching force to the materials to be welded. The pneumatic cylinder moves the electrode weld tip to one of at least three known positions. These hard stop positions include fully retracted, fully extended and a mid-point position. After welding with the weld tip in its fully extended position, the piston rod returns to a mid-point position rather than its fully retracted position. This reduces cycle time between welds. To accomplish this design parameter, normally a three position pneumatic cylinder having a fixed mid-point position is used in the spot weld gun.
Typically, the specific weld application dictates the design of the weld gun configuration. In other words, weld guns are usually custom made to fit the particular welding application. Several considerations must be addressed in custom designing a weld gun. Design parameters include the gap between the electrode weld tips, the stroke of the movable electrode, the hard positions of the electrode weld tip and the cycle time. Thus, depending upon the weld application, a custom weld gun is designed around a selected three position pneumatic cylinder. Accordingly, in an assembly line environment there may be several different weld gun configurations in operation at any one time. The obvious disadvantage of custom designed weld guns is the total lack of standardization and interchangeability.
Additionally, when a weld parameter changes in a conventional weld gun system, usually the weld gun must be disassembled and redesigned for the new welding parameter. For example, if it is desired to change the weld tip mid-point position, a new pneumatic cylinder must be substituted for the existing cylinder. During this retrofit, the weld gun must be taken out of the assembly line resulting in production down time. Attempts to avoid this disadvantage include providing weld guns having multiple pneumatic cylinders. However, the disadvantage of multiple cylinder weld guns is the increase in weight and complexity.
Accordingly, it would be desirable to provide a weld gun configuration having a standard design that can be readily interchanged with other weld guns. It is also desirable to provide a weld gun that is freely programmable to provide for any desired weld tip positioning and/or pressure.
The present invention is a fully programmable servo-pneumatic modular spot weld gun having closed-loop position and pressure control and a method for controlling the position of a weld gun welding electrode. The weld gun generally includes a weld gun base module, a pneumatic cylinder with a movable piston having a first welding electrode, a J-shaped gun arm having a first end attached to the base module and a second end having a second welding electrode and a controller for providing closed-loop positioning and pressure control of the first welding electrode.
The pneumatic cylinder includes an internal positioning encoder for detecting piston position and transmitting piston position information to the controller to control movement of the piston through a proportional flow control valve and a proportional pressure regulator. Preferably, the internal positioning encoder is a linear potentiometer fitted within the piston rod of the pneumatic cylinder to accurately determine the extension and retraction of the piston rod.
The closed-loop positioning control allows for use of a standard pneumatic cylinder having a known stroke and cylinder bore. Thus, the weld gun is adapted for releasable attachment of the pneumatic cylinder and the gun arm, whereby the pneumatic cylinder and the gun arm may be interchanged with other pneumatic cylinders and gun arms. Additionally, the weld gun base module components are made from high strength machined aluminum to reduce weight.
In a method for controlling the position of a spot weld gun weld tip, one of a plurality of binary bit position sequences corresponding to a different weld tip position are inputted to a controller that compares the inputted sequence to the actual position of the weld tip as detected by an internal positioning encoder. Based on the comparison, the controller activates a control valve of a pneumatic cylinder to extend or retract the weld tip. Preferably the bit sequence comprises four binary bits representing sixteen possible weld tip locations.
By manipulating the bit sequences and depending on the input address of the controller, other welding parameters such as weld pressure, equalizer pressure and tip dampening can be controlled. For example, a method of controlling weld pressure at the weld tip includes the steps of inputting one of a plurality of binary bit pressure sequences to a controller wherein each pressure sequence corresponds to a different pressure applied at the weld tip. The pressure sequence is compared with the actual weld tip pressure detected by a pressure regulator and, based on the comparison, the controller activates the control valve of the pneumatic cylinder to increase or decrease pressure applied at the weld tip. Similarly multiple weld tip positions may be programmed into the controller to provide a tip dampening feature to the weld gun.
For a better understanding of the present invention, reference is made to the following detailed description to be read in conjunction with the accompanying drawings and its scope will be defined in the appended claims.